The Cerenkov cone...
● First try was right – very flat shock.
cos = 1
n
Which atmospheric molecules are important for Cerenkov radiation?
● In 1937 Tamm/Frank developed a Cerenkov theory based on classical electrodynamics.
● In 1940 Ginzburg made quantum treatment and found only small corrections.
● In the classical picture, emission from a polarizable medium stems from the dielectric constant which depends on density and nuclear charge Z.
At 589 nm, 1 atmosphere pressure, 0 degrees Celsius:
Nitrogen (N2): 78% n=1.000298 Oxygen (O2): 21% n=1.000271 Argon (Ar) 1% n=1.000281 Water vapur (H20): ~1% n=1.000256 Carbon dioxide (CO2): 0.04% n=1.000449
Total n=1.000292
n= ≈
dW
dl ~∫1− 1
n2 2 d
Similar dielectric properties --> contribution roughly proportional to abundance.
More details on pair production
● Can happen only in the vicinity of a nucleus which serves as a necessary momentum absorber.
● Cross section is zero below 2me=1.02 MeV, rises to an asymptotic plateau which is reached at about 1 GeV. This limiting cross section depends on Z according to:
p=28 9
Z23h/2 c2
mec22 [ln 183 Z1/3−2
7 ]
Nitrogen Z=7 0.38 barn
Oxygen Z=8 0.49 barn
Argon Z=18 2.30 barn 1 MeV 1 GeV
E
L= 1
n =1.6710−2414
10−410−24 ≈1 km
Mean free path for gamma ray over 1 GeV:
Photomultiplier tube
Quantum efficiency: 1-10%
Response time : 1-15 ns
Noise: Low
Dynamic range: ~3 magnitudes CCD
Quantum efficiency: 90% in red, 70% in blue.
Response time: s
Noise: High if uncooled
Dynamic range: up to 10 magnitudes
Each mirror collects about 10,000 photons per flash. With the small pixel sizes of a CCD (micrometer) a CCD camera would:
● Have a too small field of view
● Put all photons in different pixels --> over-resolution.
Whats the difference between PMTs and CCDs?
How is the background from cosmic ray electrons managed?
● Up to about 1 GeV, 1% of cosmics rays are electrons.
● But the abundance then starts to drop and is only 0.1% at 100 GeV where ground based gamma astronomy starts. At higher energies the flux falls
well below the gamma ray flux, also as they come from all directions and not from the source.
● The reason is that ultrarelativistic electrons lose energy by synchrotron radiation in the galactic magnetic fields, and few remain very energetic by the time they get to earth.
